`AssertionError` with nested TypeVar

[cdce8p]

Crash Report

Encountered this issue while testing the current master branch with Home Assistant (with all requirements installed).
Bisected it to #14095.

/CC: @ilevkivskyi

Traceback

  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/types.py", line 553, in serialize
    assert not self.id.is_meta_var()
AssertionError

Full traceback

  File "/home/runner/work/ha-core/ha-core/venv/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/__main__.py", line 15, in console_entry
    main()
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/main.py", line 95, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/main.py", line 174, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/build.py", line 193, in build
    result = _build(
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/build.py", line 276, in _build
    graph = dispatch(sources, manager, stdout)
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/build.py", line 2900, in dispatch
    process_graph(graph, manager)
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/build.py", line 3288, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/build.py", line 3411, in process_stale_scc
    graph[id].write_cache()
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/build.py", line 2481, in write_cache
    new_interface_hash, self.meta = write_cache(
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/build.py", line 1571, in write_cache
    data = tree.serialize()
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/nodes.py", line 379, in serialize
    "names": self.names.serialize(self._fullname),
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/nodes.py", line 3779, in serialize
    data[key] = value.serialize(fullname, key)
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/nodes.py", line 3716, in serialize
    data["node"] = self.node.serialize()
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/nodes.py", line 1034, in serialize
    "type": None if self.type is None else self.type.serialize(),
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/types.py", line 1322, in serialize
    data["args"] = [arg.serialize() for arg in self.args]
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/types.py", line 1322, in <listcomp>
    data["args"] = [arg.serialize() for arg in self.args]
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/types.py", line 2016, in serialize
    "arg_types": [t.serialize() for t in self.arg_types],
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/types.py", line 2016, in <listcomp>
    "arg_types": [t.serialize() for t in self.arg_types],
  File "/home/runner/work/ha-core/ha-core/venv/lib/python3.9/site-packages/mypy/types.py", line 553, in serialize
    assert not self.id.is_meta_var()
AssertionError

I did add some debugging calls before the AssertionError. The serialization for this object failed

{
  '.class': 'TypeVarType', 'name': '_ZhaEntityT',
  'fullname': 'homeassistant.components.zha.core.registries._ZhaEntityT',
  'id': 13429, 'namespace': '', 'values': [], 'upper_bound': {
    '.class': 'TypeType', 'item': 'homeassistant.components.zha.entity.ZhaEntity'
  }, 'variance': 0
}

To Reproduce

Unfortunately I wasn't yet able to narrow it further to a short code sample.

However, it is easily reproducible with Home Assistant

• Check out Home Assistant
• Install attrs==21.2.0 or types-attrs==19.1.0 (only dependency required to reproduce the error it
• Run mypy homeassistant/components/zha/core

Your Environment

• Mypy version used: mypy 1.0.0+dev.a2477ff0d0cb751f27a2b38d27ce6572ead03451
• Mypy configuration options from mypy.ini (and other config files): see Home Assistant repo
• Python version used: 3.9
• Operating system and version: Github actions -> Ubuntu-20.04

[ilevkivskyi]

It looks like attrs is not important here, a simple repro for the crash:

[case testIncrementalNestedGenericCallableCrash]
from typing import TypeVar, Callable

T = TypeVar("T")

class B:
    def foo(self) -> Callable[[T], T]: ...

class C(B):
    def __init__(self) -> None:
        self.x = self.foo()
[out]
[out2]

[cdce8p]

Can confirm #14148 does resolve the issue. Thanks @ilevkivskyi for the quick fix!